Conventionally, silicon-on-quartz (SOQ), silicon-on-glass (SOG) and silicon-on-sapphire (SOS) wafers that are collectively referred to as SOI wafers have been devised. Applications of these wafers to projectors, high-frequency devices and so forth have been expected because of the insulation and transparency of handle wafers (quartz, glass, sapphire). Such an SOI wafer is produced by bonding a handle wafer to a silicon wafer (donor wafer).
The conventional SOI fabrication technique includes two main types of bonding processes.
One is called the SOITEC process. In this process, a silicon wafer (donor wafer) into which hydrogen ions have been implanted at room temperature in advance is bonded to a wafer (handle wafer) which serves as a support wafer. Then, the wafers thus bonded are heated at a high temperature (around 500° C.) to generate a large number of fine bubbles, called microcavities, in the ion-implanted interface, and to separate the silicon wafer. Consequently, a silicon film is transferred to the handle wafer.
The other is called the SiGen process. In this process, both a surface of a handle wafer and a surface of a donor wafer into which hydrogen ions have been implanted in advance in the same manner as in the above process, are subject to a plasma treatment so as activate the surfaces. Then, the wafers are bonded to each other, and thereafter the donor wafer is mechanically separated at the hydrogen ion-implanted interface.
The SOITEC process, however, has a drawback because the high-temperature heat treatment is performed after the bonding. Specifically, when the silicon wafer is bonded to the handle wafer that is typified by quartz or sapphire, the heat treatment induces wafer cracking attributable to a large difference in thermal expansion coefficients between the wafers.
Meanwhile, in the SiGen process, although the surface activation treatment allows the donor and handle wafers to have higher bonding strength upon being bonded in comparison with the SOITEC process, the bonded wafers need to be heated at 250° C. or higher. As a result of the heat treatment, the SiGen process has problems of: wafer damaged due to a difference in thermal expansion rate between the bonded wafers; and introduction of an untransferred portion in the transferred silicon film. These problems are attributed to the fact that the temperature rise increases the bonding strength at the bonding interface, but simultaneously causes warpage, detachment, and the like of the bonded wafers because these bonded wafers are different kinds. This prevents bonding of the surfaces of the wafers from progressing uniformly on the surfaces.
Heretofore, the SOS bonding technique has been studied, and a film-formation method has been proposed in which room temperature bonding, low-temperature heat treatment, grinding, and high-temperature heat treatment (900° C. or higher) are performed in this order to complete the bonding (Non-Patent Document 1). Even by this method, however, it is turned out that multiple misfit dislocations and cracks are included in the obtained silicon film layer. A presumable reason for these problems is that the silicon and sapphire substrates are bonded with such a weak bonding that stress generated during the temperature rise and fall causes the bonded surfaces to be displaced from each other. Moreover, since the silicon film is formed by grinding and polishing the silicon substrate, uniformity of the thickness of this film is considerably poor.
Furthermore, the method by Non-Patent Document 1 cannot comprise the steps of implanting hydrogen ions into a donor wafer in advance and using a thus-formed implanted interface as a transfer interface. The reason is that, in the method by Non-Patent Document 1, “the temperature (900° C. or higher) at which the bonding is completed” is much higher than “the temperature (400° C. to 600° C.) at which fine microcavities formed by the hydrogen ion implantation are enlarged for detachment” (Non-Patent Document 2).
A method typical of SOI production, called the SOITEC process (Non-Patent Document 2) comprises steps of: increasing the temperature of bonded wafers up to around 500° C., transferring a film to a handle wafer from a donor wafer into which hydrogen ions have been implanted in advance, and then increasing the temperature further up to 900° C. or higher to complete the bonding. This method has no bonding problem since the material of the handle wafer (silicon) is the same as that of donor wafer (silicon) (i.e., made of the same material).